By Laszlo Balogh ABSTRACT
The main purpose of this paper is to demonstrate a systematic approach to design high performance gate drive circuits for high speed switching applications. It is an informative collection of topics offering a “one-stop-shopping” to solve the most common design challenges. Thus it should be ofinterest to power electronics engineers at all levels of experience. The most popular circuit solutions and their performance are analyzed, including the effect of parasitic components, transient and extreme operating conditions. The discussion builds from simple to more complex problems starting with an overview of MOSFET technology and switching operation. Design procedure for ground referenced andhigh side gate drive circuits, AC coupled and transformer isolated solutions are described in great details. A special chapter deals with the gate drive requirements of the MOSFETs in synchronous rectifier applications. Several, step-by-step numerical design examples complement the paper.
MOSFET – is an acronym for Metal Oxide Semiconductor Field Effect Transistor and it is the keycomponent in high frequency, high efficiency switching applications across the electronics industry. It might be surprising, but FET technology was invented in 1930, some 20 years before the bipolar transistor. The first signal level FET transistors were built in the late 1950’s while power MOSFETs have been available from the mid 70’s. Today, millions of MOSFET transistors are integrated inmodern electronic components, from microprocessors, through “discrete” power transistors. The focus of this topic is the gate drive requirements of the power MOSFET in various switch mode power conversion applications. sourcing and sinking sufficient current to provide for fast insertion and extraction of the controlling charge. From this point of view, the MOSFETs have to be driven just as “hard”during turn-on and turn-off as a bipolar transistor to achieve comparable switching speeds. Theoretically, the switching speeds of the bipolar and MOSFET devices are close to identical, determined by the time required for the charge carriers to travel across the semiconductor region. Typical values in power devices are approximately 20 to 200 picoseconds depending on the size of the device. Thepopularity and proliferation of MOSFET technology for digital and power applications is driven by two of their major advantages over the bipolar junction transistors. One of these benefits is the ease of use of the MOSFET devices in high frequency switching applications. The MOSFET transistors are simpler to drive because their control electrode is isolated from the current conducting silicon,therefore a continuous ON current is not required. Once the MOSFET transistors are turned-on, their drive current is practically zero. Also, the controlling charge and accordingly the storage time in the MOSFET transistors is greatly reduced. This basically 1
The bipolar and the MOSFET transistors exploit the same operating principle. Fundamentally, both type of transistors arecharge controlled devices which means that their output current is proportional to the charge established in the semiconductor by the control electrode. When these devices are used as switches, both must be driven from a low impedance source capable of
eliminates the design trade-off between on state voltage drop – which is inversely proportional to excess control charge – and turn-off time. As aresult, MOSFET technology promises to use much simpler and more efficient drive circuits with significant economic benefits compared to bipolar devices. Furthermore, it is important to highlight especially for power applications, that MOSFETs have a resistive nature. The voltage drop across the drain source terminals of a MOSFET is a linear function of the current flowing in the semiconductor....